import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * Description:
 * User:何人亮
 * Date:2024-10-11
 * 15:33
 */
public class Test {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        String s=sc.nextLine();
        System.out.println(Find(s));
    }
    public static Character Find(String s){
        int[] arr=new int[256];
        for (int i = 0; i < s.length(); i++) {
            arr[s.charAt(i)]++;
        }
        for (int i = 0; i < s.length(); i++) {
            if(arr[s.charAt(i)]==1){
                return s.charAt(i);
            }
        }
        return null;
    }
}
